Great to hear that you've played.
So you'll know that nobody at the site will give a stuff how you obtain whatever you bring, whether it's a black RIF or day-glo pink IF, as long as you transport and use it sensibly.
As above, the issuing of a UKARA number is entirely up to your local site. We can't tell you what they'll say: you - by which I mean your adult co-conspirator - will have to ask them.
Since we're in the law section, I'll anticipate another question.
Yes, your friendly adult can purchase an IF ("two tone") without a UKARA. As above, Patrol Base red is pretty nice looking, and IMO works even better on a tan gun.
Yes, they can gift or loan it to you. Not sell.
No, you cannot legally modify it into a RIF by removing the paint or painting over it. That's an offence:
https://www.legislation.gov.uk/ukpga/2006/38/section/36
However, there is a defence to that offence, and it does not have a minimum age limit:
https://www.legislation.gov.uk/ukpga/2006/38/section/37
Be responsible and sensible, is what I'd emphasise above all.
https://www.legislation.gov.uk/uksi/2007/2606/regulation/7/made
7.—(1) For the purposes of section 38(3)(b) of the 2006 Act and paragraph 6(3)(b) of Schedule 2 to that Act, a colour is to be regarded as unrealistic for a real firearm only if it is a colour specified in paragraph (2) or if the imitation firearm is made of transparent material.
(2) The colours specified in this paragraph are—
(a) bright red;
(b) bright orange;
(c) bright yellow;
(d) bright green;
(e) bright pink;
(f) bright purple; and
(g) bright blue.
Eagle eyed readers will note that there's no definition of "bright". Aside, some "BB" sites sell white guns as IFs, which is a bit naughty. However, that's their concern, not the purchaser's.
. Even more if you use the search function 
